In the development of internal combustion engines, it is constantly sought to minimize fuel consumption and reduce pollutant emissions. Fuel consumption is a problem, especially in Otto-cycle engines. The reason for this lies in the principle of the working process of the traditional Otto-cycle engine which is operated with a homogeneous fuel-air mixture, in which the desired power is set by varying the charge of the combustion chamber, that is to say by means of quantity regulation. By adjusting a throttle flap which is provided in the intake tract, the pressure of the inducted air downstream of the throttle flap can be reduced to a greater or lesser extent. For a constant combustion chamber volume, it is possible in this way for the air mass, that is to say the quantity, to be set by means of the pressure of the inducted air. This also explains why quantity regulation has proven to be disadvantageous specifically in part-load operation, because low loads require a high degree of throttling and a large pressure reduction in the intake system, as a result of which the charge exchange losses increase with decreasing load and increasing throttling.
One approach for dethrottling the Otto-cycle working process is to utilize direct fuel injection. The injection of the fuel directly into the combustion chamber of the cylinder is considered to be a suitable measure for noticeably reducing fuel consumption even in Otto-cycle engines. The dethrottling of the internal combustion engine is realized by virtue of quality regulation being used within certain limits. With the direct injection of the fuel into the combustion chamber, it is possible in particular to realize a stratified combustion chamber charge, which can contribute significantly to the dethrottling of the Otto-cycle working process because the internal combustion engine can be leaned to a great extent by means of the stratified charge operation, which offers thermodynamic advantages in particular in part-load operation, that is to say in the lower and middle load range, when only small amounts of fuel are to be injected.
Direct injection is characterized by an inhomogeneous combustion chamber charge which is not characterized by a uniform air ratio but which generally has both lean (λ>1) mixture parts and rich (λ<1) mixture parts. The inhomogeneity of the fuel-air mixture is also a reason why the particle emissions known from the diesel engine process are likewise of relevance in the case of the direct-injection Otto-cycle engine, whereas said emissions are of almost no significance in the case of the traditional Otto-cycle engine.
There is relatively little time available for the injection of the fuel, for the mixture preparation in the combustion chamber, specifically the mixing of air and fuel and the preparation of the fuel within the context of preliminary reactions including evaporation, and for the ignition of the prepared mixture.
The resulting demands placed on the mixture formation relate not only to the direct-injection Otto-cycle engine but basically to any direct-injection internal combustion engine, and thus also to direct-injection diesel engines. The internal combustion engine to which the present invention relates is very generally a direct-injection internal combustion engine. For the direct injection, a fuel supply system is required which is capable of building up, in the fuel to be injected, the high pressure required for the direct injection. Therefore, the fuel supply system of a direct-injection internal combustion engine according to the prior art is equipped with at least one high-pressure pump. As a high-pressure pump, use is generally made of a piston pump in which a piston which is displaceable in translational fashion between a bottom dead center and a top dead center oscillates during the operation of the pump for the purposes of fuel delivery, in order to draw in fuel from the low-pressure side during a suction stroke and to pump, that is to say deliver, said fuel to the high-pressure side during a delivery stroke. For the regulation of the fuel volume flow, a valve unit is commonly provided by means of which the high-pressure pump is supplied with fuel from a fuel reservoir.
Depending on the conditions presently prevailing in the fuel, in particular the temperature and the pressure, a greater or lesser fraction of the fuel may evaporate, that is to say change from the liquid phase into the gaseous phase, in particular during the suction stroke. This generally leads to a malfunction of the high-pressure pump, because, owing to the gaseous fuel that is present, the pump cannot build up the high pressure required for the direct injection. Rather, the piston, which oscillates during the operation of the pump, compresses the gaseous fuel phase without delivering the demanded fuel quantity.
The delivered fuel quantity does not correspond to the demanded fuel quantity and is generally neither predictable nor reproducible. In some cases, it is even the case that fuel is no longer delivered at all, that is to say the fuel delivery to the cylinders is stopped entirely. In one example, the presence of fuel vapors at the high pressure fuel pump can result in a precipitous drop in direct injection fuel rail pressure, causing the engine to stall.
In addition, if the direct injection fuel rail pressure falls below a minimum desired direct injection pressure, it can result in unpredictable fuel injection masses. The fuel metering error may result in torque errors as well as undesirable exhaust soot emissions.
Against the background of that stated above, it is an object of the present invention to provide a direct-injection, supercharged internal combustion engine where the issues relating to the evaporation of fuel during the course of the fuel delivery can be overcome.
In one example, the issues described above may be overcome by a direct-injection, supercharged internal combustion engine having at least one cylinder, in which each cylinder is equipped with an injection apparatus for the direct injection of fuel into the cylinder, for the purposes of supplying fuel to the at least one cylinder, a fuel supply system is provided which comprises a high-pressure side and a low-pressure side, and the fuel supply system is equipped with at least one high-pressure piston pump which comprises a piston displaceable in translational fashion between a bottom dead center and a top dead center and which comprises a pressure chamber of variable volume, an inlet side and an outlet side of the high-pressure piston pump being connectable to the pressure chamber, and the displaceable piston jointly delimiting the pressure chamber with variable volume in such a way that a displacement of the piston causes a change in the volume Vchamber of the pressure chamber, which internal combustion engine is distinguished by the fact that the high-pressure piston pump is equipped with at least one movable actuation element which jointly delimits the pressure chamber such that a movement of the actuation element causes a change in the volume Vchamber of the pressure chamber, whereby the high-pressure piston pump is provided with a variable compression ratio εpump.
In one example, the high-pressure piston pump has a variable compression ratio εpump. This is realized using at least one movable actuation element which jointly delimits the pressure chamber of the high-pressure piston pump. By movement of the actuation element, the compression volume Vc can be changed, that is to say varied, whereby a variable compression volume εpump can be realized.
As used herein, the compression volume Vc is the volume that the pressure chamber has when the piston is at top dead center. The physical feature whereby the movable actuation element jointly delimits the pressure chamber is, in the context of the present invention, to be interpreted to mean that the movable actuation element either directly delimits the pressure chamber, that is to say is itself acted on by fuel, or else indirectly delimits said pressure chamber, that is to say is not itself acted on by fuel. The latter requires the provision of at least one intermediate element, for example a diaphragm, which is arranged between the fuel and the actuation element.
By reducing the size of the compression volume Vc, the compression ratio εpump can be increased, and the maximum pressure that can be realized by means of the pump can be increased in accordance with demand. In this way, evaporation of fuel can be counteracted, and/or evaporated fuel situated in the pressure chamber can be liquefied again.
This has the advantageous effect that a malfunction of the high-pressure pump can be prevented, and the pump is capable of building up the high pressure required for the direct injection. The delivered fuel quantity consequently corresponds to the demanded fuel quantity, is predictable and reproducible.
In this way, by using a high pressure piston pump having a variable compression ratio piston, issues relating to the evaporation of fuel from the high pressure pump during the course of direct fuel injection can be overcome.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.